1. Field of the Invention
The present invention relates generally to a pixel circuit and driving method for organic light-emitting diodes and, more particularly, to a pixel circuit and driving method for active matrix organic light-emitting diodes, and a display using the pixel circuit, which effectively and accurately control the brightness of organic light-emitting diodes and overcome gradient non-uniformity attributable to pixel-to-pixel mismatch.
2. Description of the Related Art
In recent years, technology for forming a Thin Film Transistor (TFT) on a substrate has been widely developed, and the development of applications related to an active matrix type display is being carried out. In particular, a TFT that employs a polysilicon film has a higher electric field effect mobility than a TFT that employs a conventional amorphous silicon film, so that the former TFT can operate at high speed. As a result, pixel control that has been conventionally carried out by a driving circuit outside a substrate can be carried out by a driving circuit formed on the same substrate as pixels.
Such an active matrix display has attracted attention because it has many advantages, such as decreased manufacturing cost, a decrease in the size of displays, increased yield, and higher throughput, that can be achieved by integrating a variety of circuits and devices with each other on the same substrate.
Currently, research into an active matrix Electro Luminescence (EL) display, including EL devices as self-light-emitting devices, is being actively carried out. The EL display is also referred to as an Organic Light-emitting Diode (OLED) display, and an active matrix OLED display is abbreviated to an AMOLED display.
The organic display is a self-light-emitting type display unlike a Liquid Crystal Display (LCD). Each of the EL devices is constituted such that an EL layer is disposed between a pair of electrodes. When electrons and holes are injected into an organic light-emitting layer formed between a first electrode (negative electrode), that is, a cathode, and a second electrode (positive electrode), that is, an anode, the injected electrons and holes are combined so as to form pairs and, therefore, an exciton is generated. The generated exciton falls from an excited state to a ground state. In this manner, the EL element emits light.
Such an OLED operates by applying a Direct Current (DC) bias of 2 to 30 Volts. The luminescence of the OELD may be controlled by adjusting the voltage or current applied to the anode and the cathode. The relative amount of light generated from the OLED is referred to as a gray level. In general, the OLED operates optimally when operating in a current mode. The optical output thereof is further stabilized by constant current driving compared to constant voltage driving. This is different from many other display technologies that generally operate in a voltage mode. Accordingly, active matrix displays that use OLED technology require a specific pixel structure to provide a current mode.
In the AMOLED that is a matrix address type, a plurality of OLEDs is typically formed on a single substrate and arrayed in the form of regular grid pattern groups. Several OLED groups that form the column of a grid may share a common cathode or a cathode line with each other. Several OLED groups that form the row of a grid may share a common anode or an anode line with each other. A predetermined group of individual OLEDs emit light when the cathode and anode lines are simultaneously activated. Each OLED group within a matrix may form a pixel for display, and each OLED generally serves as a sub pixel or a pixel cell.
The OLED has excellent characteristics, such as a wide field of view, high-speed response, and high contrast, so that they can be used for the pixel of a graphic display, a television image display or a surface light source, can be formed on a flexible, transparent substrate, such as a plastic substrate, can be manufactured very thin and light, and can provide good color. For these reasons, the OLED is expected to be the next generation Flat Panel Display (FPD).
Furthermore, the OLED can represent three colors: RED (R), Green (G) and Blue (B), has low power consumption because a backlight is not required compared to an LCD that is already well known, and provides excellent color, thus attracting attention as a device for a next generation full color display.
FIG. 1 has been disclosed in U.S. Pat. No. 6,781,567, and is one of the fundamental pixel structures for implementing conventional Time Ratio Gray (TRG).
The conventional pixel structure has the problem of an addressing time that is considered the most important problem with respect to implementing the TRG. That is, effective light-emitting time available becomes small in a given frame period in proportion to the increase of the gray scale because a frame time is divided into sub frame times and data programming must be performed for each pixel whenever the operation of each sub frame is performed so as to represent an arbitrary gray-scale, so that it is difficult to implement high level gray scale in the conventional structure.
FIGS. 2a and 2b are schemes disclosed in the Society for Information Display (SID) 2003 and SID 2004, and the pixel circuit of FIG. 2a is disadvantageous in that shoot-through current occurs at the time of the operation of a Complementary Metal-Oxide-Silicon (CMOS) inverter, and a control signal for controlling light-emitting time is added.
In FIG. 2b, the shoot-through of the pixel circuit of FIG. 2a has been eliminated, but there are disadvantages in that a separate control line is still needed and it is difficult to implement gray-scale uniformity when a transistor T5 exhibits different characteristics between pixels.